This invention relates generally to ultra-high vacuum systems, and, more particularly, to in situ getter pumps used in semiconductor manufacturing systems.
There are a number of processes which require ultra-high vacuum levels of, for example, 10.sup.-7 to 10.sup.-12 Torr. For example, high vacuum physics machines such as cyclotrons and linear accelerators often require a vacuum of the order of 10.sup.-8 -10.sup.-12 Torr. Also, in the semiconductor manufacturing industry, ultra-high vacuums of approximately 10.sup.-7 -10.sup.-9 Torr are often required in semiconductor processing equipment.
Several pumps are often used in series or parallel to achieve ultra-high vacuum levels within a chamber. A mechanical (e.g. oil) pump is often used to reduce the pressure within a chamber to approximately 30-50 millitorr. These are often referred to as "high pressure" pumps since they only pump relatively high pressure gasses. Then, a high- or ultra-high vacuum pump, such as a molecular pump, cryopump, turbo pump, or the like, is used to reduce the pressure to approximately 10.sup.-7 -10.sup.-9 Torr. These are often referred to as "low pressure" pumps since they pump low pressure gasses. The pump-down time for a particular chamber can range from minutes to hours to days depending upon such factors as the size of the chamber, the capacity of the pumps, the conductance from the chamber to the pumps, and the desired final pressure.
In certain ultra-high vacuum applications, getter pumps have been used in conjunction with the aforementioned mechanical, molecular, and cryopumps. A getter pump includes getter materials comprising metals or metal alloys which have an affinity for certain non-noble gases. For example, depending upon the composition and temperature of the getter material, getter pumps have been designed which preferentially pump certain non-noble gases such as water vapor and hydrogen.
For example, getter pumps provided by SAES Getters, S.p.A. of Milan, Italy, typically include getter material encased in a stainless steel container. Getter pumps can operate from ambient temperatures to about 450.degree. C., depending upon on the species of gas to be pumped, the getter composition, etc. A preferred getter material for prior art SAES getter pumps is St 707 getter material (which is an alloy of Zr--V--Fe) and which is produced by SAES Getters, S.p.A. of Milan, Italy. Another such material is St 101.TM. getter alloy, also available from SAES Getters, S.p.A., which is an alloy of Zr--Al. Some of these prior art getter pumps can be considered in situ pumps in that they are disposed within the high vacuum physics machines.
Some present getter pump designs employ getter devices comprising metal ribbons coated with a powdered getter material such as the St 707 and St 101.TM. getter alloys just described. The coated ribbons are pleated in a concertina fashion to increase the ratio of exposed surface area to the volume occupied by the coated ribbon, and to increase adsorption of desired gasses. Such pumps are manufactured by SAES Getters, S.p.A., and sold under the trade name SORB-AC.RTM.. In addition, recent designs have employed disk-shaped substrates coated with getter material powders. Designs using coated substrates have a drawback in that the total amount of getter material available for sorption is limited to the nominal surface area of the getter device substrate.
It is has been suggested that getter, pumps be provided for semiconductor processing equipment. For example, in an article entitled "Non-Evaporable Getter Pumps for Semiconductor Processing Equipment" by Briesacher, et al., and published in Ultra Clean Technology 1(1):49-57 (1990), it is suggested that any application which uses getters to purify processed gases for semiconductor processing can also utilize non-evaporable getter pumps for in situ purification and for the selective pumping of impurities.
The aforementioned Briesacher reference discloses that there are two possible operating scenarios for the use of getter pumps in a sputtering system, which is a type of semiconductor processing equipment. The first is the addition of the getter pump to the system to operate in parallel with conventional pumps (e.g. mechanical and cryopumps) of the system. In this scenario, the operation of the system is not modified in any way, and the getter pump merely serves as an auxiliary pump to lower the partial gas pressure of certain components of the residual gas in the chamber. The second scenario requires filling the chamber to a pressure in the range of 3.times.10.sup.-3 to 6.times.10.sup.-3 Torr, stopping the argon flow into the chamber, and sealing the chamber. The getter pump is then said to act as an "in situ" purifier for the argon. However, as discussed below, the pump is not truly "in situ" in that the active material is not within the volume of the processing chamber.
In a typical sputtering system, a noble gas (usually argon) is pumped into a chamber and a plasma is created. The plasma accelerates positively charged argon ions towards a negatively charged target, thereby causing material to become dislodged and to settle on the surface of the wafer. Getter pumps are well adapted for use with sputtering systems, since the only desired processing gas is a noble gas which is not pumped by the getter pump. Therefore, the getter pump can remove impurity gases from a sputtering chamber without affecting the flow of the noble gas required for the sputtering process.
The Briesacher reference was primarily an academic analysis of the practicality of using non-evaporable getter pumps in semiconductor processing equipment. Therefore, very little practical application of the theory is disclosed. Furthermore, while the Briesacher article uses the term "in situ" to describe a scenario for the use of a getter pump, it is clear from the description that the getter pump is external to the chamber and is considered "in situ" only in the sense that when the chamber is sealed and when no argon is flowing into the chamber, the volume within the getter pump can be considered to be connected to the chamber volume. According to the analysis presented by Briesacher, a valve must be placed between the getter containment vessel and the main chamber to protect the getter from atmospheric exposure that would deteriorate the getter and require additional regenerations. Such protection is imperative with the strip-type getters discussed in the Briesacher reference. Thus, the getter described by Briesacher is not truly "in situ" in that the getter pump surfaces are within a volume that is connected to the chamber volume through a restrictive throat, which greatly limits the conductance between the chamber and the pump. By "conductance" it is meant herein the ease with which a fluid (gas in this instance) flows from one volume (e.g. the processing chamber) to another volume (e.g. the pump chamber). Conductance is limited by the aperture size between the two chambers, which is typically the cross-sectional area of the throat of the cryopump.